In order to read out a stored value, e.g. a bit value, in an electronic memory, usually either a reference signal is used, which lies between the two states representing the ‘0’ and the ‘1’, or a complementary bit is used. When the complementary bit is used, two memory cells or memory cell elements are brought to mutually complementary states. The state ‘0’ can then be represented for example by the combination ‘0’, ‘1’, and the state ‘1’ by the opposite combination ‘1’, ‘0’. During read-out, the two states then need only be compared with one another; a reference signal can be dispensed with.
FIG. 1A to FIG. 1D illustrate the states as frequency distributions of a respective state value (current I in FIG. 1A, FIG. 1C, FIG. 1D and resistance R in FIG. 1B; generally it is possible to use e.g. Vt, R, I, t for the state value) which are encountered in memory cells that have been programmed into the two respective memory states (designated here by 1 and 0, respectively). The frequency distributions usually exhibit a broad distribution. The fact that the two distributions do not overlap, but rather are separated from one another by a gap (designated as read window), makes it possible, on the basis of the state value—either in relation to a reference value (Iref in FIG. 1C) arranged in the read window or in the comparison of two complementary state values with one another—, to unambiguously assign a memory value to a read cell. This is accomplished all the more reliably and rapidly, the larger the read window.
The read window can be characterized by a factor k=(b−a)/a, which is also designated as relative read window. In this case, a is the highest value that is expected or encountered for the distribution lying closer to zero, and b is the lowest value that is expected or encountered for the distribution lying further away from zero.
Hitherto, as is illustrated in FIG. 1C, factors of more than 6 have been customary.
In recently developed memory devices (e.g. MRAM) the read window and thus the gap between the complementary bits or between the bit and the reference bit is extremely small. This is illustrated in FIG. 1D for an exemplary MRAM memory for which the factor k=0.05.
At the same time, however, access times for reading out a bit are also intended to be reduced further and further.
Hitherto no sense amplifier has been able to read out within an acceptable time (e.g. <20 ns) given such a small read window.